1. Field of the Invention
The invention relates to electrospray ionization of dissolved substances at atmospheric pressure in the ion source of a mass spectrometer.
2. Description of the Related Art
A “chip” here is defined as a miniaturized device produced by microsystem engineering and usually having several permanently bonded layers of semiconductor materials, glasses, ceramics, metals or plastic materials. A multi-nozzle spray chip is a linear or two-dimensional arrangement of several miniaturized electrospray nozzles, spaced several hundred micrometers apart, with suitable feeds for the spray liquid and for auxiliary gases, and with suitable electrodes for the attracting and guiding voltages.
For a spray system with n nozzles operating in parallel, the ion current increases as √n at a constant total flow rate, as is described in the publication “A Micro-Fabricated Linear Array of Electrospray Emitters for Thruster Applications” by L. F. Veláquez-Garcia et al., J. Micromech. Systems 15, pp. 1260-1271, 2006. Multi-nozzle systems are therefore a means to increase the total ion yield.
The document US 2011/0147576 A1 (E. R. Wouters et al.) can be considered to be the closest prior art. Here the spray nozzles of a chip are surrounded, either individually or all together, by a flow of sheath gas which envelops the jet of sprayed droplets. The chip does not have a permanently connected counterelectrode to generate the attracting field, nor does it have a special means of guiding the sheath gas beyond the tip of the spray nozzle. The document provides an in-depth discussion of the prior art.
In the publication “Integrated out-of-plane nanoelectrospray thruster arrays for spacecraft propulsion”, R. Krpoun and H. R. Shea, J. Micromech. Microeng. 19 (2009), the spray nozzles are covered by a shared counterelectrode which has an individual opening for each spray nozzle. It does not have any sheath gas nozzles, however.
The document US 2012/0217389 A1 (Y. Zheng et al.) also describes a multi-nozzle system on a chip which has a permanently integrated counterelectrode with openings for each spray nozzle; but here too, no sheath gas flows are used.
The increase in the total ion yield as √n stated above refers to the total number of ions produced, which is important for the jet engines used in space travel. For mass spectrometric applications, however, the only aspect of interest is increasing the yield of analyte ions from analyte molecules which are dissolved in the liquid. With so-called “nanospraying”, this yield is almost 100 percent for those analyte molecules which can be protonated at all (cf. U.S. Pat. No. 5,504,329 A; M. Mann and M. Wilms, 1996). But for this nanospraying, the flow of spray liquid is limited to tiny flow rates of between 10 and 100 nanoliters per minute. If one succeeded in multiplying the nanospraying in a spray chip with n nozzles, and transferring the analyte ions produced into the vacuum with a high yield, in a similar way to nanospraying, it would be possible to obtain an n-fold number of analyte ions, with a likewise n times higher liquid flow.
In view of the foregoing, there is a need to provide a multi-nozzle system on a chip which allows all the nozzles to spray uniformly with the lowest possible ion losses. When connected to a chromatograph, it should, on the one hand, be possible to supply all the spray nozzles with the analyte molecules of a temporally short substance peak as simultaneously as possible and, on the other hand, “peak parking” should be possible. The objective is also to provide an arrangement which is adapted to the multi-nozzle system and which enables the ion current generated to be transferred into the vacuum system of a mass spectrometer with as few losses as possible.